Optical amplifiers are commonly used in optical communication systems. One of the parameters that is important in characterizing an optical amplifier is amplified spontaneous emission (ASE) noise in the presence of an optical signal. The amplified spontaneous emission noise represents a noise signal that is generated within the optical amplifier and is amplified by the amplifier. The ASE noise typically has a wider bandwidth than the optical signal. The measurement of ASE noise is important in determining the noise figure of an optical amplifier as well as ASE buildup in communication systems, where ASE noise can limit performance.
The output of an optical amplifier includes a narrow band optical signal and broader band noise generated within the amplifier. When no signal is present, the amplifier generates and amplifies noise. However, when an optical signal is present, the output noise level is reduced in comparison with the noise level in the absence of an optical signal due to amplifier gain reduction. The gain reduction depends on the amplitude of the optical signal. Thus, in order to accurately characterize amplifier performance, the ASE noise must be measured at an optical signal level and wavelength that corresponds to normal operation.
A known technique to perform ASE measurement at the actual signal wavelength is called pulse-recovery technique. The tunable laser source is gated on and off with a fast (&lt;1 .mu.s) fall time. The gated-on-time needs to be long enough such that the erbium-doped fiber amplifier (EDFA) stabilizes to its steady state for the input signal. Once the EDFA is in its steady state, the switch rapidly extinguishes the signal incident on the EDFA. Immediately after the signal is gated off, the ASE level at the amplifier output will be comparable to the true EDFA ASE level without the deleterious effects of the stimulated emission (SE) generated by the laser source. The ASE transient is recorded either with an optical spectrum analyzer (OSA) or in conjunction with an oscilloscope connected to the analog output of the OSA. For the portion of the ASE transient missed after the signal was gated off, extrapolation can be used to determine the desired ASE power density.
As the source signal is switched on, the EDFA output momentarily peaks and then returns to its steady state power level. As the source signal is switched off, the EDFA output signal drops and then the EDFA ASE power rises to its value when no signal is present. The actual ASE power density is found by extrapolating the ASE transient response immediately after the time when the source was switched off.
The disclosed technique requires two highly blocking optical switches for the incident and the outgoing laser beams of the amplifier having a short switching time and switching both beams synchronously with high accuracy.
As such a switching unit with the required accuracy is not available there is a need for a method and an apparatus by which the required accuracy can be obtained.